General objectives of the present invention include improving an enhancing brushless regenerative servomechanisms incorporating high-frequency switching control of polyphase stator currents synchronized to rotor position, to achieve greater motor efficiency and power-to-weight ratio, by eliminating high-speed core loss, and greater versatility, ease of application, and usefulness, with less mechanical complexity, quiescent power, and cost, by providing various new feedback and current control circuits.
Specific goals include providing a switching controller having pulse current sensors that detect, without quiescent power, instantaneous stator current, for pulse control of stator current having sub-microsecond loop response time, with signal processing circuits to derive from polyphase sinusoidal transducer signals a continuous lagless speed feedback for motion control having fast dynamic response, with rotation discriminator and up/down counter means for providing digital position feedback to facilitate zero-error speed control and digitally commanded position control, with a digital position readout to facilitate self-programming of sequential digital position commands, and with analog interpolation for improved dynamic response and position control resolution; and to provide a brushless coreless ultra-efficient polyphase synchronous motor having integrated Hall-effect transducers, that does not incur core loss and has maximized stator conductor volume in the magnetic field of the rotor to minimize copper loss, and has a permanent-magnet rotor with closed ironless magnetic paths to minimize stray loss and weight; and to also provide various signal interface combinations that facilitate, without a tachometer or shaft position encoder, brushless motor and controller combinations which provide complete servomechanisms, able to perform a wide range of ultra-efficient regenerative motion control tasks for a variety of useful applications.
My U.S. Pat. No. 4,085,355 for a Variable-Speed Regenerative Brushless Electric Motor and Controller System describes improvements to the prior art and establishes a specialization of the art closely related to the present invention.
Accordingly, the prior art is described and compared with the present invention herein with reference to said patent.
A prominent feature of said patent is a switching current controller and system combination that facilitates use of a polyphase synchronous machine which does not have a brush commutator; and thus no commutator friction, wear, need for periodic maintenance, spark hazard, radio frequency interference, and power dissipation therefrom. Another prominent feature is regenerative reversing and braking; whereby electric power is generated while achieving fully controlled deceleration and braking, the machine can provide full torque continuously while the controller draws only a small fraction of full-load current whenever a mechanical overload prevents rotation, there are no current surges, and permanent-magnet rotors are never subjected to excessive demagnetizing fields because stator current is always controlled within prescribed limits.
Said patent describes current control loops each having an inductively coupled current sensor in series with an associated stator winding, for providing a continuous current feedback signal which is compared with a synchronized amplitude control signal by an operational amplifier (hereinafter referred to as an op-amp) to control stator current by varying duty-cycle of power switching transistors. The required sensor bandwidth includes zero frequency. A drawback of such current control loops is their relatively slow stable response time of a few hundred microseconds. Also, its current sensor is relatively complex and needs a few hundred milliwatts quiescent power supplied above a minimum operating voltage that necessitates an under-voltage lockout to prevent excessive uncontrolled current during power connection and other low supply voltage situations.
Said patent also describes a speed sensor that provides a signal proportional to pulse rate by averaging pulses derived from polarity transitions of the transducer signals, in combination with other circuits, for varying rotor excitation inversely to speed and for limiting speed. Such pulse averaging circuits require a substantial filter lag to avoid unacceptable signal ripple, and are therefore not suitable for servomechanism speed and position control requiring fast response; moreover, the pulse rate can indicate erroneous speed substantially higher than actual if the rotor dithers about a polarity crossover and also if the transducer signal is noisy near crossovers.
The polyphase synchronous machines described in said patent have ferromagnetic cores that variously incur hysteresis and eddy current losses in core laminations subjected to alternating magnetic fields. Core losses therefrom generally increase exponentially with rotor speed according to an exponent of 1.5 to 3 or more, so the machines have high core losses at high rotor speed, especially those with permanent-magnet rotors, because their flux is not reduced at high speed. By way of example, a 12-pole machine with a 6000 revolutions-per-minute (rpm) rotor speed has an electrical frequency of 600 Hz, so its core loss compared to nominal 60 Hz operation would be greater by a factor of about (600/60).sup.3 or 1000 times. Moreover, they also incur core loss due to an inductively coupled high-frequency switching ripple component of stator current; and the inductance of their stator windings is too high to permit effective filtering by low-loss series inductance. Their high stator inductance also results in substantial reactance at high rotor speed that necessitates high supply voltage to accommodate high-speed drive, which increases core losses even more, adds to controller cost and power loss, and hinders effective and efficient regeneration. Further, cogging caused by the reluctance torque of ferromagnetic core machines can be troublesome in servomechanism applications. Additionally, a polyphase transducer, which is not included in conventional machines, must be suitably coupled to the rotor, which generally requires gearing. Furthermore, in fast response speed control servomechanism applications, a tachometer must also be mechanically coupled thereto, for providing speed feedback without excessive filter lags. And in position control applications, a tachometer and shaft position encoder mechanically coupled thereto would generally be needed.
To circumvent the above drawbacks and limitations, the present invention provides complete servomechanisms having none of the drawbacks or limitations and needing none of the mechanically coupled elements described above, without sacrificing any of the advantageous features described; so it can better meet a wide range of special requirements for diverse useful applications.
A prime example is a high-precision digital position control servomechanism that does not require a tachometer or shaft position encoder, having analog interpolated digital position feedback and analog speed feedback for fast dynamic response with regenerative damping, a digital readout for self-programming sequential digital position commands, and variable speed control between consecutive commanded destinations. These capabilities would be useful in industrial robots and machine tools.
Another prime example is a speed control servomechanism, including analog speed feedback and digital speed error integration and correction, combined with analog interpolation of digital position feedback, for providing fast dynamic response and zero speed error relative to a command pulse rate with analog interpolation between pulses. It would be useful in a variety of propulsion applications, turntables, disc and reel drives, multi-camera filming and projecting, rolling mills, and the like.
A potentially important example is a flywheel power storage system, required to have ultra-high efficiency, low quiescent power, very fast dynamic response, and negligible rotor loss, as it operates continuously at variable and very high rotor speeds, magnetically levitated in an evacuated inclosure. The electromechanical storage battery thereby provided would be especially useful as an undergrounded power storage system for solar and wind-powered point-of-use generating plants.
Other important examples include propulsion and regenerative braking for electric cars, buses, trucks, utility vehicles, rail vehicles, mobile and installed lifts and conveyances, wheelchairs and other motive devices for physically handicapped persons, plus a broad array of instrument servos and the like.
In a sailing ship, operating with lead-acid batteries that also ballast a fin keel, it could provide superb auxiliary propulsion and automatically charge the batteries from propeller rotation when sailing under wind power.
The present invention provides a wide range of advantages and enhancements to each of the above diverse examples, as well as to many others.
In accordance with the objectives and goals exemplified above, specific improvements to the controller, motor, and combinations thereof include:
New current sensors provide instantaneous current feedback pulses proportional to inductively coupled current thru the power switching transistors.
New pulse control means, responsive to the synchronized amplitude control signals and to the current feedback pulses, initiate bi-phase pulses and dictate the duration of each pulse, to control on/off time ratios of the power switching transistors; whereby the current feedback pulses are each compared to an associated synchronized amplitude control signal to dictate turn-off times for the associated transistors. This configuration provides stator current control having sub-microsecond loop response times, using rugged and reliable current sensing means that do not required quiescent power and can thus operate satisfactorily with no under-voltage lockout. Its restricted duty-cycle facilitates transformer-coupled drive to the power transistors.
New analog speed sensor means, responsive to the instantaneous frequency of the transducer signals, provide a continuous and lagless speed feedback.
New rotation discriminator means, responsive to the polarities of the transducer signals, provide feedback pulses indicating direction and amount of rotation.
New analog interpolation means provide continuous lagless linearly variable position feedback for digitally and pulse command position control.
A new low-cost circuit derives the absolute value of its input signal.
New signal interface means responsive to a command pulse rate provide signals to control speed with zero error.
New signal interface means, responsive to digital position commands, provide signals for precise position control; and, in a self-programming mode, provide digital readouts of positions reached relative to a reference and consecutively from one to the next.
New pulse queuing means present queued command and feedback pulses, with a prescribed minimum interval therebetween, to an up/down counter, for precise bidirectional speed and position control by a bidirectional variable command pulse rate.
A new low-cost circuit provides a linear speed command from a sawtooth waveform interpolation of command pulse rate.
A new rotor holding permanent-magnet disks and end-disks provides a closed magnetic field pattern therebetween without iron.
New stator rings, juxtaposed in alternation therebetween, one containing, in addition to its polyphase stator windings, corresponding Hall-effect transducers and flux-collection means therefor, conduct polyphase stator current and provide transducer signals that vary sinusoidally with rotor angle and have waveforms matching the electro-motive-force (emf) voltage of the corresponding stator winding.
A distinct stator geometry, molded within a non-ferromagnetic matrix, does not incur core loss or cogging torque, and facilitates low-resistance windings by maximizing their copper volume in the axial magnetic field of the rotor, thereby minimizing copper loss which accounts for practically all of the power dissipation in the motor.
A new flywheel power storage combination is enhanced by the motor, whose rotor incurs virtually zero heat dissipation and thus can spin freely in a vacuum inclosure with minimal heat transfer, and by the controller, which required negligible idling power and whose sub-microsecond response times prevent current surges due to supply and load transients on the dc power lines connected thereto.